As an AA EXP and UA 1K and recent CO PLT and DL Gold who has spent a lot of time trying to maximize each of these programs, I want to add my voice to the above posters saying AA has the best program. As an EXP, AA has the best international upgrades and the highest domestic upgrade success rate of any of those programs 100,000 mile levels. AA also has above average award availability and the best phone agents (EXP desk).

IF the AA route network can work for you, I think the choice is obvious, stick with AA. If you leave AA, you will be giving up the richest FF program for perhaps other benefits (better routes for your needs or preferred plane type or preferred partner airlines to redeem on).